one other compelling reason to have a shelter...if you are sick (contagious type) it is better to tent/hang by yourself out of respect for other hikers.
I remember we caught up with a section hiker last year in the 100 mile wilderness. when we caught up to her, she was in a shelter. It was starting to rain so we got in the same shelter. She was coughing/hacking/wheezing and informed us she was sick. We were in no real hurry so for the next 2-3 nights when we stopped at whichever shelter, she would inevitably show up and ask if we could "squeeze in one more" meanwhile coughing and sneezing her lungs out. Really rude if you ask me...she had her own tent but was apparently too lazy to set it up. (Not too sick to put it up as she was well enough to hike the same miles we did.) and she wasn't a kid either, probably in her early 50's. Our trail name for her became Typhoid Mary. There was enough in our group to fill the shelter so we didn't exactly exclude her but it may have been the most selfish, rude display of behavior I witnessed on the entire journey.
I went by the shelter guidelines...if it said 6 people, I didn't try to get in one but I wouldn't "squeeze one more in" either (unless it was one of the few that seemed to be off on the capacity estimate in a conservative manner). For the most part I avoided them unless it was weather related.